Energy Savings Sash Weights with Tubes + Foam vs. Removal + Foam?
I recently read an interested study on historic windows energy improvement (link below). The study proposed keeping the window sash weights and putting them inside PVC pipes (probably OD 1-7/8″) and then filling the remaining cavity with polyurethane expanding foam. Thereby maintaining the function of the weights and improving the insulation.
In the study (pg. 33 section 3.1.2) they accidently found that 90% of air leakage improvement from retrofitting a historic double hung window is through the window weight pockets and only about 10% improvement in air leakage is through the sashes. They used densely packed fiberglass in the weight pockets, which leaks more air than polyurethane foam.
They also found (pg. 66) that the tube/weight combo with polyurethane foam improved R-value of the window by 8% in still air (from 1.29). They also showed that adding weatherstripping + an IGU ONLY to the sashes improved R-value from 1.29 to 2.09 (pg. 8). (the author clarified they did add an IGU).
However, they did not measure R-value improvement by just removing the weights and filling the pocket with polyurethane foam to compare against the 8% improvement of the weight/tube/foam combo. So my question is if the cost of sash spring balances is $120 per window what is payback time for weight removal/foam vs. weight/tube/foam. The weight box is 5″ thick and 3″ wide.
Link to referenced study:
Microsoft Word – #2008-M1-024 Final report 24JAN2011.docx (dc.gov)
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